What is the extension of a neuron that transmits messages to other neurons or cells?

Cells within the nervous system, called neurons, communicate with each other in unique ways. The neuron is the basic working unit of the brain, a specialized cell designed to transmit information to other nerve cells, muscle, or gland cells.

Neurons are cells within the nervous system that transmit information to other nerve cells, muscle, or gland cells. Most neurons have a cell body, an axon, and dendrites. The cell body contains the nucleus and cytoplasm. The axon extends from the cell body and often gives rise to many smaller branches before ending at nerve terminals. Dendrites extend from the neuron cell body and receive messages from other neurons. Synapses are the contact points where one neuron communicates with another. The dendrites are covered with synapses formed by the ends of axons from other neurons.

Illustration by Lydia V. Kibiuk, Baltimore, MD; Devon Stuart, Harrisburg, PA

Neurons are cells within the nervous system that transmit information to other nerve cells, muscle, or gland cells. Most neurons have a cell body, an axon, and dendrites. The cell body contains the nucleus and cytoplasm. The axon extends from the cell body and often gives rise to many smaller branches before ending at nerve terminals. Dendrites extend from the neuron cell body and receive messages from other neurons. Synapses are the contact points where one neuron communicates with another. The dendrites are covered with synapses formed by the ends of axons from other neurons.

The brain is what it is because of the structural and functional properties of interconnected neurons. The mammalian brain contains between 100 million and 100 billion neurons, depending on the species. Each mammalian neuron consists of a cell body, dendrites, and an axon.

The cell body contains the nucleus and cytoplasm. The axon extends from the cell body and often gives rise to many smaller branches before ending at nerve terminals.

Dendrites extend from the neuron cell body and receive messages from other neurons. Synapses are the contact points where one neuron communicates with another. The dendrites are covered with synapses formed by the ends of axons from other neurons.

When neurons receive or send messages, they transmit electrical impulses along their axons, which can range in length from a tiny fraction of an inch (or centimeter) to three feet (about one meter) or more. Many axons are covered with a layered myelin sheath, which accelerates the transmission of electrical signals along the axon. This sheath is made by specialized cells called glia. In the brain, the glia that make the sheath are called oligodendrocytes, and in the peripheral nervous system, they are known as Schwann cells.

The brain contains at least ten times more glia than neurons. Glia perform many jobs. Researchers have known for a while that glia transport nutrients to neurons, clean up brain debris, digest parts of dead neurons, and help hold neurons in place. Current research is uncovering important new roles for glia in brain function.

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It is clear that most of what we think of as our mental life involves the activities of the nervous system, especially the brain.  This nervous system is composed of billions of cells, the most essential being the nerve cells or neurons.  There are estimated to be as many as 100 billion neurons in our nervous system!

A typical neuron has all the parts that any cell would have, and a few specialized structures that set it apart.  The main portion of the cell is called the soma or cell body.  It contains the nucleus, which in turn contains the genetic material in the form of chromosomes.

Neurons have a large number of extensions called dendrites.  They often look likes branches or spikes extending out from the cell body.  It is primarily the surfaces of the dendrites that receive chemical messages from other neurons.

One extension is different from all the others, and is called the axon.  Although in some neurons, it is hard to distinguish from the dendrites, in others it is easily distinguished by its length.  The purpose of the axon is to transmit an electro-chemical signal to other neurons, sometimes over a considerable distance.  In the neurons that make up the nerves running from the spinal cord to your toes, the axons can be as long as three feet!

Longer axons are usually covered with a myelin sheath, a series of fatty cells which have wrapped around an axon many times.  These make the axon look like a necklace of sausage-shaped beads.  They serve a similar function as the insulation around electrical wire.

At the very end of the axon is the axon ending, which goes by a variety of names such as the bouton, the synaptic knob, the axon foot, and so on (I do not know why no one has settled on a consistent term!).  It is there that the electro-chemical signal that has travelled the length of the axon is converted into a chemical message that travels to the next neuron.

Between the axon ending and the dendrite of the next neuron is a very tiny gap called the synapse (or synaptic gap, or synaptic cleft), which we will discuss in a little bit.  For every neuron, there are between 1000 and 10,000 synapses.

The action potential

When chemicals contact the surface of a neuron, they change the balance of ions (electrically charged atoms) between the inside and outside of the cell membrane.  When this change reaches a threshold level, this effect runs across the cell's membrane to the axon.  When it reaches the axon, it initiates the action potential, which is a rapidly moving exchange of ions.

The surface of the axon contains hundreds of thousands of miniscule mechanisms called ion channels. When the charge enters the axon, the ion channels at the base of the axon allow positively charged ions to enter the axon, changing the electrical balance between inside and outside.  This causes the next group of ion channels to do the same, while other channels return positive ions to the outside, and so on all the way down the axon.

In this little diagram, the red represents the positive ions going into the axon, while the orange represents positive ions going out.  The action potential travels at a rate of 1.2 to 250 miles per hour!

This is, of course, over-simplified, but enough for our purposes.  But if you are interested in a little more detail, click here!

When the action potential reaches the axon ending, it causes tiny bubbles of chemicals called vesicles to release their contents into the synaptic gap.  These chemicals are called neurotransmitters.  These sail across the gap to the next neuron, where they find special places on the cell membrane of the next neuron called receptor sites.

Types of Neurons

While there are many different kinds of neurons, there are three broad categories based on function:

1.  Sensory neurons are sensitive to various non-neural stimuli.  There are sensory neurons in the skin, muscles, joints, and organs that indicate pressure, temperature, and pain.  There are more specialized neurons in the nose and tongue that are sensitive to the molecular shapes we perceive as tastes and smells.  Neurons in the inner ear are sensitive to vibration, and provide us with information about sound.  And the rods and cones of the retina are sensitive to light, and allow us to see.

2.  Motor neurons are able to stimulate muscle cells throughout the body, including the muscles of the heart, diaphragm, intestines, bladder, and glands.

3.  Interneurons are the neurons that provide connections between sensory and motor neurons, as well as between themselves.  The neurons of the central nervous system, including the brain, are all interneurons.

Most neurons are collected into "packages" of one sort or another, sometimes visible to the naked eye.  A clump of neuron cell bodies, for example, is called a ganglion (plural: ganglia) or a nucleus (plural: nuclei).  A fiber made up of many axons is called a nerve.  In the brain and spinal cord, areas that are mostly axons are called white matter, and it is possible to differentiate pathways or tracts of these axons.  Areas that include large number of cell bodies are called gray matter.

© Copyright 2003, 2009 C. George Boeree

What are the extensions of neurons called?

What extend from the neuron cell body and receive messages from other neurons?

How do neurons transmit messages from one cell to another?

Which part of the neuron sends messages to other neurons?